Thermal dye transfer technology is known for its ability to provide an excellent, continuous-tone, full-color image. In thermal dye transfer printing, an image is formed on a receptor element by selectively transferring a dye to a receptor element from a dye donor element placed in momentary contact with the receptor element. It is a characteristic of the thermal dye transfer process (sometimes also referred to in the art as "sublimation transfer") that a dye diffuses without a carrier vehicle from the dye donor element directed by a thermal source, typically a thermal print head, which consists of small electrically heated elements. These elements transfer image-forming material from the dye donor element to areas of the dye receptor element in an image-wise manner.
Thermal dye transfer systems have advantages over other thermal transfer systems, such as chemical reaction systems and thermal mass transfer systems. In general, thermal dye transfer systems offer greater control of gray scale than these other systems, but they have problems as well. One problem is lack of release between the dye donor and receptor elements. This leads to unwanted mass transfer (e.g., blocking or sticking of the dye coat to receptor) during dye transfer. This problem has often been addressed by the addition of dye-permeable release coatings applied to the surface of the dye receptor layer. Additionally, materials are required for use in the receptor layer having suitable dye permeability, mordanting properties, adhesion to the substrate, and long term light and thermal stability.
Thermal mass transfer printing has also been employed in the art to provide thermal images. Although lacking continuous-tone imaging capability, thermal mass transfer is capable of generating a bright, dense, solid half-tone image. The term "thermal mass transfer" refers to thermal imaging processes in which a colorant is transferred from a donor element to the surface of a receptor element by action of a thermal source as described above, but without sublimation of the dye or colorant. Often the colorant is contained within a binder that is also transferred in the process, such as disclosed, for example, in U.S. Pat. Nos. 4,839,224 and 4,822,643. Also, the colorant may be present in a binderless construction as disclosed in U.S. Pat. No. 4,985,321. Thermal mass transfer processes may generally be carried out on colorants that do not exhibit measurable thermal diffusion in the image-receiving layer (e.g., pigments, metals, etc.), although colorants that do exhibit diffusion may be used. In contrast, pigments are not generally useful in the thermal dye diffusion process.
One drawback with thermal mass transfer has sometimes been the inability of the thermal mass transfer donor element to adequately adhere to the receiving layer, thereby leading to incomplete or no mass transfer of colorant into the receiving layer which is necessary to produce an adequate image. As a result, special receiving or receptor layers are required.
Polyvinyl chloride derivatives and copolymers have been used in thermal dye transfer receptor elements because of their advantageous properties. For example, U.S. Pat. No. 4,853,365 discloses that chlorinated polyvinyl chloride, used as a dye image receptor, has good dye solubility and high dye receptivity. Similarly, vinyl chloride/vinyl acetate copolymers have also been used in thermal dye transfer receptor elements as disclosed in Japanese Kokai Application Nos. 29,391 (1990) and 39,995 (1990). Japanese Kokai Application No. 160,681 (1989) discloses dye acceptance layers containing polyvinyl chloride-polyvinyl alcohol copolymers and Japanese Kokai Application Nos. 43,092 (1990); 95,891 (1990); and 108,591 (1990) disclose dye receptor layers containing a hydroxy-modified polyvinyl chloride resin and an isocyanate compound.
U.S. Pat. No. 4,990,485 discloses a heat-transfer image-receiving sheet containing a substrate and a dye-receiving layer that is composed of a graft copolymer having at least one grafted polysiloxane segment. The backbone of the copolymer chain may be vinyl chloride-containing copolymers including vinyl chloride-vinyl acetate and vinyl chloride-acrylic acid copolymers. At column 15, lines 9-17, the patent discloses the use of a vinyl chloride/n-butyl acrylate/glycidyl methacrylate/vinyl-modified polymethyl methacrylate copolymer which is grafted with stearyl alcohol.
Generally, polyvinyl chloride-based polymers are photolytically unstable, decomposing to form hydrogen chloride, which in turn degrades the image-forming dyes. This has necessitated the extensive use of UV stabilizers and compounds that neutralize hydrogen chloride.
Other materials have been used in such receptor elements as well. For example, U.S. Pat. No. 4,897,377 discloses a thermal transfer printing receiver element containing a supporting substrate coated on at least one surface with an amorphous polyester resin. Laid-open European Patent Application No. 133,012 (1985) discloses a heat transferable element having a substrate and an image-receiving layer thereon containing a resin having an ester, urethane, amide, urea, or highly polar linkage and a dye-releasing agent, such as a silicone oil, being present either in the image-receiving layer or as a release layer on at least part of the receptor layer. Laid-open European Patent Application No. 133,011 (1985) discloses a heat transferable element based on imaging layer materials containing first and second regions, composed, respectively, of: (1) a synthetic resin having a glass transition (T.sub.g) temperature of from -100.degree. C. to 20.degree. C., and (2) a polar group; and a synthetic resin having a T.sub.g of 40.degree. C. or above.
U.S. Pat. No. 4,914,078 discloses a receiver coat containing a dye-receptive material and a thermoset, amino-modified, silicone organic epoxide-based resin.
U.S. Pat. Nos. 4,626,256 and 4,927,666 disclose an image receiving sheet containing a dye permeable releasing agent containing a reaction hardened product of an amino-modified silicone and an epoxy-modified silicone.
U.S. Pat. No. 4,910,189 discloses a thermal transfer dyesheet containing a binder that further contains a thermoset silicone composition.
U.S. Pat. No. 4,931,423 discloses a thermal dye transfer receiving layer containing a resin and a silicone oil having a concentration gradient in the receiving layer.
What is needed in the industry is a thermal transfer imaging process which possesses the advantages of both thermal dye transfer and thermal mass transfer, but not their respective disadvantages and drawbacks.